Apparatus and method for high bandwidth laser-based data communication

ABSTRACT

A laser communication system includes a first laser to generate a laser signal with femtosecond pulses. A first grating spectrally disperses the femtosecond pulses of the laser signal. A modulator converts the femtosecond pulses of the laser signal into coded words. A second grating spectrally recombines the coded words of the laser signal. A first telescope launches the laser signal. A second telescope receives the laser signal. A second laser generates a set of reference pulses. A non-linear crystal combines the set of reference pulses and the laser signal so as to create an output signal only when the laser signal and the reference pulses temporally coincide. A detector records the output.

This invention claims priority to the provisional patent applicationentitled, “Apparatus and Method for Line of Sight Laser Communications”,Ser. No. 60/068,184, filed Dec. 19, 1997.

BRIEF DESCRIPTION OF THE INVNETION

This invention relates generally to data communications. Moreparticularly, this invention relates to a technique for high bandwidthlaser-based data communications.

BACKGROUND OF THE INVENTION

Solid state fiber lasers have been developed for commercialcommunication applications. Picosecond pulses (ca 50 ps) are currentlybeing used for fiberoptic based long distance (transpacific) solitoncommunication. Much shorter pulses (<100 Fs) have been generated withfiber sources, pumped by diode lasers. Such systems have the advantagesthat they are being tailored for communications, the bandwidth that isused is at an “eye safe” wavelength, and fast modulation techniques havebeen or are being developed. There is still work to be done, but thefact that the light is confined to a narrow waveguide helps improve thespeed of modulation. The disadvantages of such systems is that they havea lossy transition from fiber to air, they have a larger diffractionangle at longer wavelengths, the minimum size beam at 20 km is about 10m, there is a low average and peak power (pJ/pulse), and a lowrepetition rate (1 to 10 MHz) exists.

Solid state Nd vanadate lasers (Nd:YVO_(—)4) could also be used incommunication systems. These diode pumped lasers produce a train ofpulses of about 5 ps duration (which could be compressed externally tothe laser to 100 fs). The repetition rate is typically 100 MHz. Theadvantage of systems of this type are high efficiency, high averagepower-up to 100 W in an infrared beam, 20 W in the green, which can befrequency-tripled to a wavelength of 355 nm. The typical pulse energy inthe green is 0.2 uJ/pulse. Pump diodes for this type of laser have along lifetime; they are actively under development and they arerelatively cost effective. In addition, these systems can providediffraction limited beams in the green and UV. For the green, theminimum spot size at 20 km is about 2 m. For the UV, the minimum spotsize at 20 km is about 1.5 m.

Solid state Cr:LiSAF Lasers (also Cr:LiSGAF and Cr:LiCAF) are anotheroption for communication systems. Tunable pulse generation in the rangeof 820 nm to 880 nm has been demonstrated. Pulse duration as short as 20fs has been obtained, but at very low average power. A maximum averagepower has been demonstrated at 1.1 W, for continuous operation, whileshort pulse operation is only 0.5 W. Advantageously, these systems haveshorter pulses directly out of the laser, without compression. Inaddition, they have a shorter wavelength. The disadvantages of thesesystems is that they have less efficient pump diodes and less averagepower.

Existing laser-based communication systems rely upon light modulationthrough electronic control techniques. Thus, the speeds of such systemsare inherently limited to the speeds of the electronic control systems.It would be highly desirable to to eliminate the speed limitation ofelectronic control systems. In particular, it would be highlyadvantageous to substitute an electronic control system with an opticalcontrol system to enhance the performance of a laser-based communicationsystem.

Existing laser-based communication systems use fiber optic channels tocommunicate information. It would be desirable to reduce the expense andcomplexity of a laser-based communication system by eliminating thefiber optic channel. The absence of a fiber optic channel would allowmore flexibility in developing communication system infrastructures.

SUMMARY OF THE INVENTION

A laser communication system includes a first laser to generate a lasersignal with femtosecond pulses. A first grating spectrally disperses thefemtosecond pulses of the laser signal. A modulator converts thefemtosecond pulses of the laser signal into a string of pulsesrepresentative of coded words. A second grating spectrally recombinesthe coded words of the laser signal. A first telescope launches thelaser signal. A second telescope receives the laser signal. A secondlaser generates a set of reference pulses. A non-linear crystal combinesthe set of reference pulses and the laser signal so as to create anoutput signal only when the laser signal and the reference pulsestemporally coincide. A detector senses and records the output. The firstlaser also generates a synchronization signal. The second laser issynchronized by the synchronization signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference should be made tothe following detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a laser communication emitter in accordance with anembodiment of the present invention.

FIG. 2 illustrates a laser communication receiver in accordance with anembodiment of the present invention.

FIG. 3 illustrates an apparatus to perform amplitude modulation inaccordance with an embodiment of the invention.

FIG. 4 is a more detailed illustration of selected components of theapparatus of FIG. 3.

FIG. 5 is a more detailed illustration, drawn to scale, of selectedcomponents of the apparatus of FIG. 3.

FIG. 6 illustrates a one-dimensional phase modulator array in accordancewith an embodiment of the invention.

FIG. 7 is a more detailed illustration of the apparatus of FIG. 6.

FIG. 8 is a more detailed illustration, drawn to scale, of selectedcomponents of the apparatus of FIG. 6.

FIG. 9 illustrates an apparatus for detecting an amplitude modulatedsignal at a remote destination.

FIG. 10 illustrates a spectral component that is processed in accordancewith an embodiment of the invention.

FIG. 11 illustrates a cross-correlation apparatus utilized in accordancewith an embodiment of the invention.

FIG. 12 illustrates a two-photon detector utilized in conjunction withthe apparatus of FIG. 11.

FIG. 13 illustrates a multiplexing apparatus utilized in accordance withan embodiment of the invention.

FIG. 14 illustrates the time combination of various channels into asingle light beam in accordance with an embodiment of the invention.

Like reference numerals refer to corresponding parts throughout thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a laser communication emitter 20 constructed inaccordance with an embodiment of the invention. The apparatus 20includes a laser source 22 that emits a laser beam 23 as a train offemtosecond (1×10⁻¹⁵ sec) pulses at a rate of 100 MHz (10 ns betweenpulses). The pulses are redirected by a mirror 24 and then spectrallydispersed by a first grating 26. Preferably, the mirror 24 isimplemented in the form of an “echelon reflector”, which consists of a“staircase” of mirrors. At each successive level, the reflecting surfaceis receded, giving the corresponding part of the beam a different delay.The mirror displacement should be equal to the delay between eachchannel times the speed of light. For an inter-channel delay of 10 ps,this corresponds to 3 mm. At each level, the beam is sent to a grating,which separates the various spectral components in the plane of thefigure.

The spectral components then pass through a lens 28 and enter amodulator 30, which converts the pulses into “words” or coded sequenceof pulses. The pulses are routed through a lens 32 and are thenspectrally recombined by a second grating 34 before being reflectedthrough a mirror 36 and being launched by a telescope 38. Asynchronization beam (not shown) from the same laser source 22 issimultaneously sent through the telescope 38.

Systems similar to the system of FIG. 1 are known in the art. However,such prior art systems do not utilize a laser source 22 with femtosecondpulses and they do not utilize the modulator 30 of the invention, whichis discussed in detail below. Thus, the prior art systems that otherwiseappear to be similar, do not achieve the high bandwidth and otheradvantages associated with the present invention.

FIG. 2 illustrates a laser communication receiver 40 constructed inaccordance with an embodiment of the invention. The apparatus 40includes a receiving telescope 41 to receive the laser signal fromemitter 20. The input signal is reflected with a mirror 42 into apolarizing beam splitter 44. The signal is then routed to a non-linearcrystal 46, where it is combined with a signal from laser 48. Laser 48applies a laser beam 49 to grating 50. The signal is then reflected bymirror 52 into the non-linear crystal 46. The signal from the laser 48is a set of pulses that operate at the same repetition rate as the inputsignal from the telescope 41. The grating 50 or prisms are used tospatially delay the reference pulses from the reference laser 48. Thenon-linear crystal 46 only generates light when the reference light andthe signal temporally coincide. Since the time of arrival of thereference light is “sloped” in the plane of the figure, the temporalsequence of the signal is transformed in a transverse pattern recordedby a fast camera or imaging array 54.

Thus, the invention provides femtosecond laser technology for very highspeed and secure point-to-point communications. The technique of theinvention provides more than a magnitude greater bandwidth than existingpoint-to-point communications technologies. This performance improvementis largely attributable to the fact that the control of the laser signalis achieved through optical means, not electronic means. Theoptical-based modulator of the invention facilitates a large increase inprocessing speed over prior art systems.

An array of microlenses may be used to focus portions of the beam intomicro-electro-optic modulators, then back to microlenses and the mainbeam. The “length” of the word that can be carried at every period isdetermined by the spectral resolution of the system. The word isrefreshed every period (10 ns). An alternative approach is to usesuccessive periods for multiplexing.

The most obvious method of detection is to repeat at the receiving endwhat is being done at the emitter. If amplitude modulation is used, itis sufficient to display and read the spectrum of each word. It is alsonecessary to use single shot cross-correlations to isolate every 50 pspulse sequence that is to be read. The detection end requires a similarsource as the emitter, which is synchronized with the emitter.

To perform the synchronization, some signal is required to identify thebeginning of each “word”. If the basic period is the pulse repetitionrate of 100 MHz, the infrared pulse may be used for synchronization, andthe visible and UV pulses may be used for the signal to be transmitted.

There are a number of benefits associated with the invention. First, theinvention provides a method of creating any wireless optical (laser)based communications link, using pulse code modulation with trains offemtosecond pulses unaffected by atmospheric diffraction (such asline-of-sight).

The invention is also advantageous because it uses optical amplifiers asrepeaters. The invention can be advantageously used along (above andbelow ground) the rights-of-way currently managed and/or owned by theutilities around the world. For example, laser signals may be passedthrough underground ducts or between powerline transmission towers. Thecommunications bandwidth, limited only by the bandwidth of the laser andthe femtosecond pulses, is several orders of magnitude greater bandwidththan conventional microwave communications systems.

The laser communications systems are extremely secure, virtuallyimpossible to “tap” into because of the unique nature of the lasersignal and the narrowness of the laser beam. The laser system can bemade inherently safe. When the emitter pulses are interrupted by anyobstacle, the system is preferably designed to shut down.

The invention and its benefits have now been fully described. Attentionpresently turns to a more detailed discussion of various components ofthe invention. Pulses transmitted from the laser 22 are shaped by themodulator 30 of the invention. The modulator 30 may be configured toperform amplitude modulation or phase modulation. In the case ofamplitude modulation, information is sent as a set of dark-bright linesimposed on the pulse spectrum. Preferably, the voltage to drive themodulator does not exceed 5V (TTL standard pulse). The modulatorpreferably responds in less than 10 ns (the time between successivepulses of the main train). Modulators are positioned to form a threedimensional stack. Each successive plane corresponds to one of thechannels to be multiplexed.

An array of Mach Zehnder modulators may be used to establish amplitudemodulation in accordance with the invention. A Mach Zehnder modulatorarray in accordance with the invention is shown in FIG. 3. Fresnellenses 60 are used to receive each spectral component and focus eachcomponent onto an optical waveguide 62. The waveguide 62 is split intotwo equal arms 62A and 62B. One arm (e.g., 62B) is surrounded byelectrodes 64. If an electric field is applied to one of the arms of theMach Zehnder interferometer using the electrodes 64, the radiation fromthe two arms interfere destructively. The two arms of the waveguide 62Aand 62B converge into a single waveguide, forming a Mach Zehnderinterferometer. Lenses 66 are used to focus the output light from theinterferometer. Using state of the art lithographic techniques, theFresnel lenses can be manufactured directly on the edge of the crystal(e.g., LiNbO₃) used as a modulator. This eliminates the need for complexalignment systems, and makes it possible to stack modulators on top ofone another.

FIG. 4 is a more detailed sketch of the device of FIG. 3 withaccompanying dimensions. Contact pods 70 are located at the sides of thesample. The ratio of the waveguide width to the spacing is chosen for 50Ohm impedance of the electrical waveguide. Rather than connecting on oneend all electrodes to a common ground, each is individually connected toa bias control in order to adjust each waveguide for maximumtransmission in the absence of a pulsed signal.

The optical waveguide of FIG. 4 is 5 microns wide and approximately 3microns deep. The portion of the waveguide between electrodes is chosenfor a π phase shift, for a TTL pulse. FIG. 4 is not to scale andtherefore does not give a sense of the proportions of the device.

FIG. 5 more accurately represents the scale of the device. As with FIG.4, only a few of the waveguides are represented in FIG. 5. The last twowaveguides (32 and 31) are shown in FIG. 5. Thereafter, only everyfourth waveguide is shown. One series (low side) of electrodes connectto contact pods 70 at the upper edge of the sample. The correspondinghigh side of the electrodes connects to contact pods 70 at the loweredge (not shown). The median in both directions is indicated by adot-dashed line. Microlenses can be made by lithography on the entranceand exit edges of the crystal, with the optic axis of the lens along thewaveguide.

An amplitude modulation device to implement the modulator 30 has nowbeen described in connection with FIGS. 3-5. Attention now turns to aphase modulation device that may be used to implement the modulator 30.In the case of phase modulation, the information is sent as a set ofphase shifts imposed on each spectral component.

A phase modulator that may be used in accordance with the invention isshown in FIG. 6. Once again, an array of microlenses 80 is used tolaunch the light into an array of waveguides 82. The microlenses 80 maybe positioned on the same substrate as the waveguides 82. The waveguidesare formed of lithium niobate. Each entering spectral component isrepresented by lines 84. Depending on the voltage applied to theelectrode 86 on the waveguide on which the light is launched, thecorresponding radiation may be shifted in phase. Phase shifted spectralcomponents at the output side of lenses 87 are shown with lines 88.

Details of the design of the 1-dimensional array of phase modulators inLiNbO₃ are shown in FIG. 7. Thirty-two channels (waveguides separated by150 microns) are designed on a 1×1 cm² substrate. In the case of a pulseof 100 fs (10 nm bandwidth) pulses at 100 Mhz centered around 800 nmresult in a bandwidth per channel of(10/64)=0.15 nm.

Two of the 32 waveguides are shown in FIG. 7 as 82A and 82B. The widthand spacing (d) of each electrode pair 86 is chosen to correspond to a50 Ohm transmission line. The length L of the electrode is such as toinduce a π phase shift on the waveguide upon application of 5 Volts.

Design considerations for the waveguide include a single mode,polarization preserving device and optimized fabrication parameters. Theelectrode design considerations include consideration of the length ofthe electrodes “L” and the separation “d” for 0-π phase change (0-10Volts). Preferably, the leads are sufficiently separated from the padsto avoid crosstalk. A scaled sketch of the electrode design is shown inFIG. 8.

As in the case of the amplitude modulator, light has to be launchedsimultaneously in all waveguides, which requires a very accuratemicrolens array design, and also extremely accurate positioning.Positioning may be achieved by piezoelectric micromanipulators. Thesecan be avoided if microlenses are made directly on the edge of thecrystal. That is, the microlenses may be fabricated on the samesubstrate as the interferometers.

The foregoing discussion related to individual components of the systemof FIG. 1. Attention presently turns to individual components of thesystem of FIG. 2. In particular, attention turns to different techniquesfor detecting the laser signals sent from the system of FIG. 1.

The detection of the amplitude modulated signal is the simplest, becauseit amounts to measuring the spectrum of the light, as gated by a 10 pspulse to select the appropriate channel. One approach is depicted inFIG. 9, where parametric gain is used to simultaneously amplify thesignal to be detected, and select the appropriate channel. The laserused at the reception for this particular amplitude modulation detectionis emitting a synchronized train of 10 ps pulses at a wavelength shorterthan that of the signal, and at the repetition rate corresponding tothat of the emission (the period of emission of each channel). Themethod of synchronization can simply be injection mode-locking—theemitter is made to send a strong pulse at that particular wavelength. Avery weak signal is needed to force another laser to operatesynchronously, and at the same wavelength (injection mode locking). The10 ps pulse serves as a pump in a parametric amplifier crystal, in whichthe signal detected by the telescope is seeded. The parametric crystalamplifies the signal only during the time gate of 10 ps. It should beremembered that in the case of “amplitude modulation” the sequence ofelectronic pulses that constitutes the useful signal of a particularchannel has been “imprinted” onto the spectrum of a fs pulse. Therefore,one has to read and decode the original message by measuring, with adetector array, the spectrum of the gated, amplified signal, as shown inFIG. 9. Many of the components of FIG. 9 correspond to components ofFIG. 2, as reflected with identical reference numerals. In FIG. 9, thenon-linear crystal 46 is a parametric amplifier crystal. The Fouriertransform of the amplified output signal has the same pattern as theoriginal pulse signal at the transmitter. The fast camera 54 of FIG. 2is replaced with a simple detector array in FIG. 9.

The delay element 49 of FIG. 9 is a layered structure, for instance astack of glass plates of different lengths, such that beams insuccessive planes are delayed with respect to each other by the timeseparation between successive channels (typically 10 ps). The signalreceived by the telescope contains all channels. The channel selectionis made by amplifying the relevant part of the main signal in aparametric amplifier. The energy to the amplification is provided by thesynchronizing laser, which is of a shorter wavelength than the laserbeam containing the modulation. In the case of amplitude modulation,detection is made simply by reading the spectrum of the light at eachlevel with a detector array 54.

Phase modulation detection techniques may also be used in accordancewith the invention. In the case of phase modulation, it is the relativedelay between each spectral component that has to be detected. Theproduct of a “line of delta functions” extracted from the referencelaser is made with the signal to be detected, in a crystal of ZnSe inwhich a photocurrent proportional to the product of the reference timesthe received signal is produced. The particular signal amplified is areplica of the signal sent into the particular channel.

As in the case of amplitude modulation, the Fourier transform of the fspulse sequence corresponding to an individual channel has to be derived.The phase of each spectral component has to be measured, which is a moredifficult challenge than just measuring the amplitude. However, sincedigital signals are being processed, there are only two values of thephase to record. In the time domain, the phase of an individualfrequency component corresponds to its delay, or time of arrival. Take,for example, 32-bit communication. At the emission, the spectrum of the100 fs pulse has been divided into 32 spikes. The frequency spacingbetween adjacent spectral components is δ=10¹³ s⁻¹/32=0.3 10 ¹²s⁻¹.

If a spectral component is given a phase shift of π, its timecorrespondent (which is a 3 ps pulse) will have a delay of one halfwavelength. The most sensitive detection is to detect the product of thepulse corresponding to the spectral component with a 100 fs referencesignal, positioning the reference pulse at the leading edge of thespectral component, as shown in FIG. 10. FIG. 10 illustrates onespectral component of the train of fs (ps) pulses that contain thesignal plotted as a finction of time. Assume that the spectral componenthas been assigned the phase π. Its inverse Fourier transform istherefore delayed by one half wavelength. Next, a reference pulse issent, timed to appear at some time during the leading edge of thesignal. On the right, the result of the nonlinear correlation betweenthe reference and either signal spectrum is shown. The contrast ismaximum if the reference is chosen along the greatest slope of thesignal.

The correlator has to be a sensitive detector, and at the same timenonlinear. Such a correlator has been designed based upon two-photonphotoconductivity in ZnSe. The sample is 100 μm thick, and of highmobility (600 cm²/volt-sec). A voltage close to breakdown voltage willbe able to sweep the photoelectrons to one electrode, beforerecombination occurs. The light is sent normal to a thin edge (0.1 mm×10 mm) of the crystal, focused by a linear array of microlenses. Thelight has a centimeter to penetrate and create carriers by two-photonabsorption. The reference pulses and signal are sent collinear into thedetector.

The grating compressor and the reference delay must be designed suchthat the energy front of the reference pulses and the spectral componentpulses (corresponding to zero phase) are parallel. The overall detectiontechnique is shown in FIG. 11. In particular, FIG. 11 illustrates adetection cross-correlator 100 and an associated two-photon detectorarray 102. The two-levels of output of the array correspond to the twovalues of the spectral phase. A phase shift of a spectral component ofthe incoming signal implies that the corresponding time signal isdelayed. A reference pulse is cross-correlated with the rising edge ofeach spectral component. The result of the cross-correlation is anindication as to whether the spectral component has been phase shiftedor not.

The two-photon detector array 102 has been fabricated on a 100 micronthick sample of single crystal ZnSe, as shown in FIG. 12. Each spectralcomponent is focused onto the edge face of the ZnSe crystal 110. Thatis, lenses 112 are used to focus the spectral components onto electrodes114. The electrodes 114 are separated by etched trenches 116. Thefocusing is chosen such that the confocal parameter of the beam iswithin the crystal length. An electric field closest to the breakdownvoltage is applied between each upper electrode and the correspondingbottom electrode, in order to “sweep away” the two-photon photocarriers.

Beams from each channel are focused into the waveguide detector by alens array similar to that used at the modulation end. The two-photoninduced photocurrent from each channel is preamplified and stored.Microlenses can be cut directly in the edge face of the ZnSe.

Attention presently turns to multiplexing operations performed inaccordance with the invention. FIG. 13 illustrates the overallarchitecture of a femtosecond time-of-flight communication system inaccordance with the invention. The synchronization beam, indicating witha short pulse the beginning of each string of fs signals, can be derivedfrom the same femtosecond laser used to provide the fs pulses into eachpulse shaper. Typically, a shorter wavelength is used forsynchronization. The system of FIG. 13 assumes several channels withdigital pulse coded modulation at 10 Ghz. A femtosecond laser isproviding a train of pulses at, for example, 100 MHz. Each femtosecondpulse is split into a large number (as many as there are channels; up to1,000 in this example) of pulses, and given an incremental delay of 10ps. Each of these pulses is used to make a compressed version of theoriginal word (spanning 10 ps instead of 10 ns). To this effect, themultiplexer consists of a stack of modulators of the type shown in FIG.3 or FIG. 6. All the various signals are then recombined together intothe beam to be emitted. The clock laser at 100 MHz is operated at twosimultaneous wavelengths. The shorter wavelength train of pulses is sentdirectly through the same telescope as used for sending out themodulated signal to the receiver.

In the multiplexer 120 of FIG. 13, each of the mutually delayed pulsesis combined with an element or word of the appropriate channel toproduce a replica of the information (or data) contained in thatchannel, but compressed in a time span of a few picoseconds (less than10). The information of each channel, compressed to less than 10 ps, isput into a temporal succession in the main beam to be sent fordetection. When detected, the 10 ns between two successive pulses isdivided into as many 10 ps channels as was sent from the emitter. Theinformation contained in each 10 ps segment is thereafter decoded toreproduce the original electronic information that was contained in thechannel.

The shorter wavelength train is used at the reception as an injection tothe local oscillator laser at the reception. Its function is mainly thatof a synchronizer pulse used to demultiplex the successive channels atthe reception.

FIG. 14 illustrates the combination of various channels into a singlelight beam. The compression of a nanosecond time sequence of pulses (theoriginal “word” in each channel) into a fs pulse train is establishedusing either the amplitude modulation or phase modulation techniquesdescribed above.

At the emitter, either in the amplitude or phase modulator mode, theLiNbO₃ modulators are stacked on top of each other, and the linearmicrolens arrays become 2-D lens arrays. The successive delays of thereference pulses that define the channel can be realized with “echelon”reflectors of the type described above.

The reception demultiplexing is the same tri-dimensional structure asthe emission. In the case of phase modulation, the successive detectorarrays made of 100 μm thick ZnSe plates are stacked on top of eachother. An echelon reflector is used for the reference pulses used in theparametric amplifier and in the cross-correlation.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention. Wellknown circuits and devices are shown in block diagram form in order toavoid unnecessary distraction from the underlying invention. Thus, theforegoing descriptions of specific embodiments of the present inventionare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed, obviously many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, to thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and their equivalents.

What is claimed is:
 1. A laser communication system, comprising: a firstlaser to generate a laser signal with femtosecond pulses characterizingdata to be transferred; a first grating to spectrally disperse saidfemtosecond pulses of said laser signal; an opto-electronic modulator toconvert said femtosecond pulses of said laser signal into time modulatedcoded words characterizing said data to be transferred; a second gratingto spectrally recombine said time modulated coded words of said lasersignal; and a telescope to launch said laser signal; a receivingtelescope to receive said laser signal; a second laser to generate a setof reference pulses; a cross-correlator to cross-correlate referencepulses from said second laser with selected spectral components of saidlaser signal to determine whether said selected spectral components havebeen phase shifted; a non-linear crystal to combine said set ofreference pulses and said laser signal so as to create an output signalonly when said laser signal and said reference pulses temporallycoincide; and a detector to record said output signal.
 2. The apparatusof claim 1 wherein said opto-electronic modulator is an amplitudemodulator.
 3. The apparatus of claim 2 wherein said amplitude modulatorincludes a plurality of waveguides formed in a waveguide substrate, eachwaveguide forming a portion of an interferometer to process a spectralcomponent of said laser signal.
 4. The apparatus of claim 3 furthercomprising lenses positioned on said waveguide substrate at the inputand output of each waveguide of said plurality of waveguides.
 5. Theapparatus of claim 3 further comprising a plurality of waveguidesubstrates arranged in a stack.
 6. The apparatus of claim 1 wherein saidopto-electronic modulator is a phase modulator.
 7. The apparatus ofclaim 6 wherein said phase modulator includes a plurality of waveguidesformed in a waveguide substrate, each waveguide forming a portion of aninterferometer to process a spectral component of said laser signal. 8.The apparatus of claim 7 further comprising lenses positioned on saidwaveguide substrate at the input and output of each waveguide of saidplurality of waveguides.
 9. The apparatus of claim 7 further comprisinga plurality of waveguide substrates arranged in a stack.
 10. Theapparatus of claim 1 wherein said second laser is synchronized with saidfirst laser by injection mode locking.
 11. The apparatus of claim 1wherein said non-linear crystal amplitude modulates said laser signal.12. The apparatus of claim 1 wherein said first laser simultaneouslygenerates said laser signal and a synchronization signal.
 13. A methodof laser-based data communications said method comprising the steps of:generating a laser signal with femtosecond pulses characterizing a datasignal to be transferred; spectrally dispersing said femtosecond pulsesof said laser signal; converting said femtosecond pulses of said lasersignal into time modulated coded words characterizing said data signalto be transferred; spectrally recombining said time modulated codedwords of said laser signal; launching said laser signal from a homelocation to a remote destination; receiving said laser signal at saidremote destination; generating a set of reference pulses;cross-correlating reference pulses from said second laser with selectedspectral components of said laser signal to determine whether saidselected spectral components have been phase shifted; combining said setof reference pulses and said laser signal so as to create an outputsignal only when said laser signal and said reference pulses temporallycoincide; and recording said output signal.
 14. The method of claim 13wherein said converting step includes the step of of amplitudemodulating said femtosecond pulses of said laser signal.
 15. The methodof claim 14 wherein said converting step includes the step of processingspectral components of said laser signal in an interferometer.
 16. Themethod of claim 13 wherein said converting step includes the step ofphase modulating said femtosecond pulses of said laser signal.
 17. Themethod of claim 16 wherein said converting step includes the step ofprocessing spectral components of said laser signal in aninterferometer.
 18. The method of claim 13 further comprising the stepof synchronizing a laser at said home location with a laser at saidremote destination by injection mode locking.
 19. The method of claim 13wherein said combining step includes the step of amplitude modulatingsaid laser signal.
 20. The method of claim 13 wherein said generatingstep includes the step of simultaneously generating said laser signaland a synchronization signal.